1. Field of the Invention
This invention relates to a developing device, and, more particularly, it is concerned with a developing device incorporated in an image forming device such as electro-photographic apparatus, etc. and for developing an electrostatic image formed on an image holding member such as a photosensitive drum, etc.
As the developing method utilizing a single or one-component developing agent (or developer or toner), there have so far been known various sorts such as, for example, the powder clouding method, in which the toner particles are used in the atomized form; the contact method, in which a toner layer of a uniform thickness formed on a developing roller consisting of a web, sheet, or the like is caused to contact with the electrostatic image holding surface to thereby effect the development; the jumping method, in which the toner layer is positioned in confrontation to the electrostatic image holding surface without contact between them, and the toner particles are caused to selectively fly toward the image holding surface due to the electric field on the electrostatic image; the magne-dry method, wherein a magnetic brush is formed by using an electrically conductive magnetic toner, and this magnetic brush is caused to contact the electrostatic image holding surface to thereby effect the development; and other methods.
Of these various developing methods, the jumping method has been known as unique. This method is effected in such an arrangement that a developer is applied on a developer holding or conveying means in a thin layer, after which an electrostatic image surface is opposed to the front surface layer of this thinly applied developer.
In this arrangement, the developer is caused to fly, due to the electrostatic force of attraction, from the developer conveying means to the electrostatic latent image surface to effect the development (vide: U.S. Pat. No. 2,839,400 and No. 3,232,190). According to this jumping method, not only is the developer not attracted to the non-image portion having therein no latent image potential, but also the developer does not contact the non-image portion, hence a favorable development completely free from the ground fogging can be realized.
In view, however, of the fact that, at the time of the development, use is made of the flying phenomenon of the toner particles due to the electric field of the electrostatic image, the visible image obtained generally lacks sharpness at its edge portion, so that the resulting image is devoid of clearness. In the case of developing a linear image, it is developed thinner in appearance than its original, and in only an image poor in its tonality (due to steep gradation (.gamma.) in the characteristic curve potential).
The assignee of the present inventors have proposed new developing method in their U.S. patent applications Ser. No. 938,494, Nos. 938,101, 938,494 and No. 58,434, all abandoned, in which sharpness and tonality of the image, which have been the problems in the conventional jumping development, are improved.
The first two developing methods have characteristics such that a single component developer is uniformly applied onto a developer conveying means to form a thin, uniform layer of developer, and this thin developer layer is opposed to the electrostatic latent image forming surface with a small gap between them so that the outer surface of the developer layer is not in contact with the latent image forming surface, and then the development is effected by extending or stretching forth the developer confronting the image portion by its electrostatic force of attraction. Since the development by these methods is done under conditions where the developer is not in contact with the non-image portion, a sharp image free from the ground fogging can be obtained.
The second two methods are constructed in the same manner as in the above two methods, in addition to which an AC bias voltage is applied as a developing bias voltage across the thin developer layer and the electrostatic image forming surface, and a gap between the electrostatic latent image surface and the developer holding means is changed with lapse of time. According to this developing method, the developer is caused to reach even the non-image portion of the electrostatic latent image at the initial stage of the development so that development can be effected at a half tone portion, while the developer is caused to arrive at the image portion alone after a lapse of time. By this developing method, image development can be performed free from fogging, having sharpness in the resulting image, and giving extremely favorable reproducibility in the half tone.
In these developing methods, it is important that a thin layer of developer be uniformly coated on the developer conveying means. While one-component magnetic developer may easily be put into practical use, because it can be relatively easily controlled by the combined use of a magnetic means, with one-component, non-magnetic developer it is difficult to obtain a stable and favorable result. It has therefore been a practice so far that, in each of the above-mentioned developing methods, the one-component non-magnetic developer is fed onto the developer conveying means such as an ordinary developing roller, etc, and then the developer is charged by a frictional charging member, after which the developer holding means is disposed in confrontation to the electrostatic latent image surface.
However, when the toner quantity to be supplied to the developing roller is not constant, and the toner is fused and coagulated onto the frictional charging member and the developing roller surface due to wear of the frictional charging member and the developing roller caused by friction between them, there inevitably takes place irregularity in the toner application, whereby the electric charge quantity for the toner charging becomes irregular. Such irregularity reflects on the copy image in the form of an image irregularity at the time of the development.
Further, when the surface of the developing roller is made irregular with a view to holding the toner particle layer thereon, or when a toner holding surface layer includes a multitude of tiny holes distributed throughout, the toner particles which have been captured in the irregular surface or in the tiny holes are not easily charged, as a result of which the toner does not contribute to the development and the copy image obtained is inferior in its developing characteristics and is low in its image density.